1. Field of the Invention
The invention relates to flowable coating compositions for producing scratch-resistant coatings (SRCs), based on multifunctional acrylates and methacrylates, and a method of coating molded articles, particularly articles comprised of plastics, such as organic glasses, with a particularly clear coating in the form of a hardened thin layer which arises from polymerization of essentially polyfunctional acrylic and/or methacrylic compounds, which coating is distinguished by high scratch-resistance even under conditions of open-air weathering, and which coating has an oil- and water-repellent action.
2. Discussion of the Background
According to German laid-open applications DE 21 64 716 and DE 24 55 715, multifunctional acrylate systems can be polymerized with thermally decomposable polymerization initiators, the usual oil soluble peroxides and azo compounds; or by initiation by irradiation, for example, by UV radiation acting in the presence of suitable photoinitiators.
Preferably UV-initiation is used, because the method can be carried out in the presence of air. According to German laid-open application DE 29 28 512, an SRC with improved adhesion results if such a UV polymerization is carried out between 70.degree. C. and the glass temperature of the plastic to be coated.
Peroxide initiation is known for production of coated, scratch-resistant plates with good weather-resistance. According to European Patent EP-B 0 245 728, the coating uses aliphatic peroxydicarbonates as peroxide initiators.
UV-hardenable SRCs formed from acrylic resins with added fluoroalkyl-group-containing (meth)acrylate comonomers are known from the following three Japanese patent publications:
Japanese Patent 63-196,667 (ref. CA 110:97285), discloses a coating for PC screens, wherein the SRC agents consist of the following, in a solvent mixture:
(a) 10-60% of (meth)acrylic acid esters of formula EQU H.sub.2 C.dbd.C(R)CO.sub.2 CH.sub.2 R.sub.f, PA1 (b) polyfunctional acrylates; PA1 A represents an alkylene group with 2-6 carbon atoms; PA1 R.sub.2 represents H or an alkyl group with 1-4 carbon atoms; and PA1 R.sub.3 represents C.sub.a H.sub.b F.sub.c, where a, b, and c are integers and a=2-30; b=0-4; c=2a+1-b. PA1 R.sub.3 represents C.sub.a H.sub.b F.sub.c, where a, b, and c are integers, and a=2-20; b=0-4; c=2a+1-b; PA1 A represents an alkylene group with 2-6 carbons atoms; PA1 R.sub.2 represents H or an alkyl group with 1-4 carbons atoms; and PA1 R.sub.3 represents C.sub.a H.sub.b F.sub.c, PA1 where a=2-20; b=0-4; c=2a+1-b; PA1 (a) .gtoreq.30 wt. % of a polyfunctional (meth)acrylate compound, PA1 (b) 0.1-20 wt. % of a fluorinated-alkyl-group-containing (meth)acrylate monomer of formula I, PA1 (c) &lt;30 wt. % of a second copolymerizable compound, and PA1 (d) customary additives. PA1 A represents an alkylene group with 2-6 carbons atoms; PA1 R.sub.2 represents H or an alkyl group with 1-4 carbon atoms; PA1 R.sub.3 represents C.sub.a H.sub.b F.sub.c,
where R represents H or CH.sub.3, and R.sub.f represents a fluoroalkyl group with 1-20 C atoms; and
Japanese Patent 60-151,601 (ref. CA 104:69,939), discloses coatings for plastic lenses comprised of polydiallyl compounds, wherein employed is a butylmethylketone solution containing polyfunctional acrylates and dihydroperfluorohexyl acrylate in a weight ratio of 55:45; and
Japanese Patent 77-105,936 (ref. CA 88:52,095), discloses coatings of polymethacrylate lenses, wherein employed is a flowable coating resin comprised of polyfunctional acrylic esters, methyl methacrylate, and 1 wt. % of hexafluoropropyl methacrylate based on the weight of the monomers. The fluorine-containing comonomers confer improved optical properties and improved surface smoothness on the hard coating of the polymethacrylate lens.
European published Application EP-A-40 923 discloses radiation-hardenable mixtures of substances which form strongly adhering polymer coatings, wherein the coating mixtures are composed of polyethylenically unsaturated and crosslinking substances, a film-forming polymer, and a polyfluorinated acrylate, wherein the fluorinated compound may particularly have the formulae EQU H.sub.2 C.dbd.C(R)CO.sub.2 (CH.sub.2).sub.1-12 R.sub.f (A)
or ##STR2##
where R represents H or CH.sub.3, R.sup.1 represents H or an alkyl group with 1-4 carbon atoms, and R.sub.f represents a polyfluorinated alkyl group with 6-20 carbon atoms. B may be, for example, N-ethyl-heptadecafluorooctane-sulfonamidoethyl acrylate.
Coating compositions for PMMA plates are disclosed in Japanese Patent 61-258,870 (ref. CA 107:135,968), composed of diacrylate, epoxyacrylate, and fluoroalkyl-group-containing methacrylates such as represented by formulas (A) and (B) in the preceding paragraph with reference to European Published Application EP-A-40 923. These compositions do not allow production of hard and scratch--resistant coatings.
Plastic molded bodies which have SRCs are particularly employed in the form of plates on building exteriors, for example, on sound-protecting walls or as glazing panels of facades, bus stop enclosure walls, advertising panels, advertising pillars, and so-called "urban furniture". In such uses the coatings may be particularly subject not only to natural soiling but to soiling of direct human origin via vandalism, e.g. graffiti. It is very costly to clean such surfaces repeatedly without at least some invasion of the surface, for example, as physical abrasion.